Electrical mask inspection

ABSTRACT

An apparatus and method for electrical mask inspection is disclosed. A scan chain is formed amongst two metal layers and a via layer. One of the three layers is a functional layer under test, and the other two layers are test layers. A resistance measurement of the scan chain is used to determine if a potential defect exists within one of the vias or metal segments comprising the scan chain.

FIELD OF THE INVENTION

The present invention relates generally to semiconductor fabrication,and more specifically to techniques for inspecting and verifying masksused in the fabrication of semiconductors.

BACKGROUND OF THE INVENTION

The capacity of integrated circuits has increased primarily as theresult of reductions in the size of features on a semiconductor chip.The lateral dimensions of features are generally defined byphotolithographic techniques in which a detailed pattern is transferredto a reactive material by shining light through a photomask or reticle.In this disclosure, the term “photomask” is interchangeable with theterm “reticle.” During the photolithography process, energy is appliedto photoresist deposited on a wafer, where the energy application iscontrolled through the use of a patterned photomask. The exposure to thewafer is made by a step and repeat procedure. In this procedure, thewafer is moved and the steppers are used to move and repeat the patternof the photomask over the wafer.

Printable defects on photomasks and reticles have historically been asource of defects that have reduced die yields. Printable defects in thephotomasks are repeated many times over the surface of a semiconductorwafer since the photomask is stepped and repeated over the wafer. Whendefects are present on the photomask, this can result in substantialyield losses. Accordingly, it is important to detect as many defects aspossible in the photomasks. Furthermore, defects can occur with repeateduse of the photomask. Therefore, it is desirable to have an improvedapparatus and method for inspection of photomasks.

SUMMARY OF THE INVENTION

One embodiment of the present invention provides a test circuitcomprises: a first endpoint (pad); a second endpoint (pad); a firstmetal layer, comprising a plurality of segments; a second metal layer,comprising a plurality of segments; a via layer, comprising a pluralityof vias wherein each via is configured and disposed to connect a segmentof the first metal layer to a segment of the second metal layer; whereinthe first metal layer comprises a segment connected to said firstendpoint and also comprises a segment connected to said second endpoint;wherein one of the layers selected from the group consisting of thefirst metal layer, the second metal layer, and the via layer is afunctional layer, and the remaining layers of said group are testlayers.

Another embodiment of the present invention provides a method fortesting an integrated circuit. The method comprises the steps of:forming a first endpoint; forming a second endpoint; forming a firstmetal layer, comprising a plurality of segments; forming a second metallayer, comprising a plurality of segments; forming a via layer,comprising a plurality of vias wherein each via is configured anddisposed to connect a segment of the first metal layer to a segment ofthe second metal layer; wherein the step of forming the first metallayer comprises forming a segment connected to said first endpoint andalso comprises forming a segment connected to said second endpoint; andwherein one of the layers formed is a functional layer, and wherein twoof the layers formed are test layers.

Another embodiment of the present invention provides a method fortesting an integrated circuit. The method comprises the steps of:forming a plurality of scan chains, each scan chain formed by the stepsof: forming a first endpoint; forming a second endpoint; forming a firstmetal layer, comprising a plurality of segments; forming a second metallayer, comprising a plurality of segments; forming a via layer,comprising a plurality of vias wherein each via is configured anddisposed to connect a segment of the first metal layer to a segment ofthe second metal layer; wherein the step of forming the first metallayer comprises forming a segment connected to said first endpoint andalso comprises forming a segment connected to said second endpoint; andwherein one of the layers formed is a functional layer, and wherein twoof the layers formed are test layers; performing a resistancemeasurement between the first endpoint and second endpoint of each ofthe plurality of scan chains; and indicating a failure if the resistancemeasurement exceeds a predetermined threshold.

BRIEF DESCRIPTION OF THE DRAWINGS

The structure, operation, and advantages of the present invention willbecome further apparent upon consideration of the following descriptiontaken in conjunction with the accompanying figures (FIGS.). The figuresare intended to be illustrative, not limiting.

Certain elements in some of the figures may be omitted, or illustratednot-to-scale, for illustrative clarity. The cross-sectional views may bein the form of “slices”, or “near-sighted” cross-sectional views,omitting certain background lines which would otherwise be visible in a“true” cross-sectional view, for illustrative clarity.

Often, similar elements may be referred to by similar numbers in variousfigures (FIGS) of the drawing, in which case typically the last twosignificant digits may be the same, the most significant digit being thenumber of the drawing figure (FIG).

FIG. 1 is a side view of a portion of a multiple layer integratedcircuit.

FIG. 2 is a top-down view of a portion of a multiple layer integratedcircuit.

FIG. 3 shows a scan chain for testing a via level.

FIG. 3B and 3C show integrated circuits which comprise multiple testcircuits.

FIG. 4 shows a scan chain for testing a metal level.

FIG. 5 shows a flowchart for calculating parameters for conducting a vialevel test.

FIG. 6 shows a flowchart for calculating parameters for conducting ametal level test.

DETAILED DESCRIPTION

FIG. 1 is a side view of a portion of a multiple layer integratedcircuit 100. At certain points within the integrated circuit, two metallayers are electrically connected by conductive vias. As shown in FIG.1, metal layer M(x) is connected to metal layer M(x−1) with a via at vialayer V(x). Furthermore, metal layer M(x) is connected to metal layerM(x+1) with via level V(x+1).

FIG. 2 is a top view of a portion of a multiple layer integrated circuit200. Metal layer M(x+1) is electrically connected to metal layer M(x)through via layer V(x+1). In general, an integrated circuit may becomprised of millions of devices, such as transistors and capacitorsinterconnected by multiple metal layers and via layers. Testingconnectivity at each “level” (e.g. M1, M2, V1, etc . . . ) of anintegrated circuit can be challenging. Embodiments of the presentinvention serve to provide an effective way to periodically check themasks corresponding to various levels. A functional level is a portionof a circuit used for a working integrated circuit, such as a DRAM,processor, or other device. A working device may have multiplefunctional metal levels, and in general, a device with N functionalmetal levels has (N−1) functional via levels.

FIG. 3 shows a test circuit (scan chain) 300 in accordance with anembodiment of the present invention. Test circuit 300 is used to testthe functional V(x+1) level photomask. To perform the test of thefunctional V(x+1) level photomask, functional M(x+1) level and M(x)levels are replaced with a test M(x+1) level and a test M(x) levelrespectively. Test pads 308 and 310 are the endpoints of test circuit300. Test circuit 300 is also referred to as a scan chain. The scanchain is used to test a portion of an integrated circuit. The scan chaincomprises two endpoints, which are pads (308 and 310), and a pluralityof metal segments (shown generally as 302 and 304) which terminate atvias (shown generally as 306). As per legend 312, segment 302(represented as a solid line) is a part of metal layer M(x+1) andsegment 304 (represented as a dotted line) is a part of metal layerM(x). Each square represents a via in level V(x+1). The M(x) and M(x+1)segments are part of test metal layers. The layer V(x+1) is a functionalvia layer (i.e. V(x+1) is a layer used in a functional integratedcircuit). In one embodiment, multiple scan chains are used to test anentire integrated circuit. In one embodiment, 24 pads may be used. Eachpad is fairly large compared to the feature size. In one embodiment, thepads are of a square shape, ranging in size from about 60 microns toabout 100 microns. During testing, a standard wafer probing diagnostictool, widely used in the industry, performs a resistance measurementbetween pad 308 and pad 310. This tests the functional via level V(x+1)by using test levels for M(x) and M(x+1). The test levels of M(x) andM(x+1) are arranged such that functional vias form a part of the scanchain. If there is a connectivity problem with one of the vias, then theresistance measured across pads 308 and 310 is larger than if theconnectivity is good. When the resistance measured across pads 308 and310 exceeds a predetermined value (e.g. 150 Ohms) a failure isindicated. In this way, it is possible to quickly determine that aconnectivity problem exists in at least one of the vias in the scanchain.

The predetermined threshold resistance depends on the properties of thescan chain. For any given technology and process, the specifiedresistance range for either a via (ohms per via) or a metal line(ohms/square) can be used to determine the resistance value above which,a reading will be considered as indicative of a circuit failure.

The maximum resistance specified for any given scan chain is computedas:

(V×Rv)+(Ms×Rs)

Where V is the number of vias, Rv is the resistance of a via, Ms is thenumber of metal squares, and Rm is the resistance of a metal square.Each metal line is considered as a plurality of metal squares for thepurposes of the resistance calculation. A longer metal line is composedof more squares than a shorter metal line. Hence the length of the metallines are considered when computing the threshold resistance.

For example, supposing the resistance of a via is 5 ohms per via, andthe resistance of a metal level is 1 ohm per square, and that aparticular scan chain has 10 vias and 100 squares of metal, theresistance threshold is computed, using the above formula, as:

10×5+100×1=50+100=150 Ohms

Measured resistances exceeding 150 ohms for this scan indicates that oneof the elements within the scan chain is outside the specified range,and is considered as an issue that warrants further investigation.

In one embodiment of the present invention, a functional device isdivided into logical subgroups. These subgroups may be based onfunctional blocks (e.g. ALU, shift registers, and memory circuits) or onvarious locations within the integrated circuit. Each scan chain is thenassociated with a particular functional area of the chip. This allowseach scan chain to test a particular functional area of the chip (e.g.ALU, shift registers, and memory circuits).

FIG. 3B shows an integrated circuit 350 comprising multiple testcircuits 352, 354, and 356. In this way, the entire number of viaswithin a level are divided among the three test circuits (scan chains).If there is a failure (indicated by a high resistance reading across thetwo pads of the scan chain), then the failure can then be isolated to asubgroup of the vias belonging to the particular scan chain, which aidsin identifying the faulty vias. Once the subgroup containing the faultyvia is identified, the defect may be further localized to a specific viaor vias by voltage-contrast techniques. In the embodiment of FIG. 3B,the functional elements under test (e.g. vias of a functional level) areassigned to a particular test circuit based on physical positioning ofthat element within the chip. Elements in the top portion of the chipare tested with test circuit 352. Elements in the middle portion of thechip are tested with test circuit 354. Elements in the bottom portion ofthe chip are tested with test circuit 356.

FIG. 3C shows an integrated circuit 360 comprising multiple testcircuits 362, 364, and 366. In this case, each test circuit isassociated with a particular functional area of the chip, meaning thateach test circuit comprises functional elements (e.g. vias or metalsegments) that correspond to a particular functional area. This allowseach scan chain to test a particular functional area of the chip. Forexample, in one embodiment, test circuit 362 may be associated with anALU, test circuit 364 may be associated with shift registers, and testcircuit 366 may be associated with memory circuits.

FIG. 4 shows a test circuit 400 for testing functional metal levelM(x+1). M(x) and V(x) are test levels. A test mask is used for M(x) andV(x), and the positions of each via in the V(x) level is arranged as soto connect all two-ended shapes of functional mask M(x+1).

Certain portions of a metal mask that have more than two endpoints, suchas an “E-shaped” segment (not shown), may be omitted from the scanchain. The test circuit 400 is evaluated in a similar manner to testcircuit 300, which is by measuring the resistance between pads 408 and410. As per legend 412, segment 402 (represented as a solid line) is apart of metal layer M(x+1) and segment 404 (represented as a dottedline) is a part of metal layer M(x). Each square represents a via inlevel V(x). The M(x) and V(x) layers are test layers, and M(x+1) is afunctional layer.

Some of the advantages provided by embodiments of the present inventioninclude the ability to detect process shifts in the fabrication process.Periodic testing (e.g. every 2500 exposures) can detect a possibleproblem with a photomask, such as possible damage or contamination.Another advantage is that this method inspects functional components,that is, parts of an actual level used in the integrated circuit, asopposed to relying solely on test patterns to perform analysis. Anotheradvantage is the ability to isolate defects to within a particularlayer, and sometimes within a particular area within the layer,depending on how many scan chains are used. Yet another advantage isreduction in “false positives.” In many integrated circuit designs,redundant vias are used to increase reliability and product yield. Ifone of a group of redundant vias fails, embodiments of the presentinvention can still register a pass on the test, since the resistancebetween pads will still be low if at least one of the group of redundantvias is intact. For the test levels, various design constraints such asmetal wiring widths and spacing can be increased, since the density isnot as important in the test levels. This simplifies the design of thetest levels for metal and vias and improves yield of the test levelsensuring maximum sensitivity for defects in the functional layer. Fortest metal levels, the width of the segments should exceed the minimumwidth of the feature size where possible, to ensure contact andelectrical continuity. In one embodiment, the test metal segments are 10to 20 percent wider than the minimum width (as indicated in FIG. 4,where M(x+1) is indicated with wider lines than M(x)). For example, ifthe minimum width of the segments is 45 nm (nanometers), then the testsegments may have a width of 50-60 nanometers.

FIG. 5 shows a flowchart 500 for calculating parameters for conducting avia level test. In process step 550, the number of vias in via levelV(x) is established as “N.” In process step 552, the number of chains iscomputed as the number of pads divided by two. In process step 554, thenumber of vias per scan chain is computed as N divided by the number ofchains. Note that the number of vias per scan chain computed in processstep 554 is intended as a guideline. It is possible to practiceembodiments of the present invention where different scan chains havedifferent numbers of vias in them, such as in the case where variousfunctional blocks are tested with different scan chains.

In one embodiment, the process for forming the scan chain to test a vialevel is as follows: Starting with a first pad (e.g. pad 308 of FIG. 3),build a metal segment to the nearest via at metal level M(x). Then builda metal segment from that via to another neighboring via using metallevel M(x+1). This process repeats, alternating metal levels with eachhop in between vias, until the complete scan chain is formed,terminating at the second pad (e.g. pad 310 of FIG. 3).

In an example, supposing there are 100 pad sets of 24 pads each within aparticular chip design and also supposing that there are 7200 Vx vias,then the calculations outlined in flowchart 500 are computed as follows:

N=7200

NumPads=100×24=2400

NumChains=NumPads/2=2400/2=1200

ViasPerChain=N/NumChains=7200/1200=6

Note that the above example is merely illustrative, and the value of Nused in the example is small compared to what would be encountered in areal application. In practice, the value N may be in excess of onemillion.

FIG. 6 shows a flowchart 600 for calculating parameters for conducting ametal level test. In process step 650, the number of distinct two-endedlines in functional metal level M(x) is established as “L.” In processstep 652, the number of chains is computed as the number of pads dividedby two. In process step 654, the number of links per scan chain iscomputed as L divided by the number of chains. Note that the number oflinks per scan chain computed in process step 554 is intended as aguideline. It is possible to practice embodiments of the presentinvention where different scan chains have different numbers of links inthem, such as in the case where various functional blocks are testedwith different scan chains.

In one embodiment, the process for forming the scan chain to test ametal level M(x+1) is as follows: Starting with a first pad (e.g. pad408 of FIG. 4), build a metal segment at level M(x) to the nearest endof the nearest segment at metal level M(x+1). Then build a via frommetal level M(x+1) to metal level M(x) at that location. Traverse to theother end of the metal segment at metal level M(x+1) and build a via tolevel M(x), then form another metal segment at metal level M(x) to thenearest end of the next segment at metal level M(x+1). This processrepeats, alternating metal levels with each hop in between vias, untilthe complete scan chain is formed, terminating at the second pad (e.g.pad 410 of FIG. 4).

As can now be appreciated, embodiments of the present invention providethe ability to conduct testing on functional areas of an integratedcircuit to evaluate the current condition of production photomasks andreticles.

Although the invention has been shown and described with respect to acertain preferred embodiment or embodiments, certain equivalentalterations and modifications will occur to others skilled in the artupon the reading and understanding of this specification and the annexeddrawings. In particular regard to the various functions performed by theabove described components (assemblies, devices, circuits, etc.) theterms (including a reference to a “means”) used to describe suchcomponents are intended to correspond, unless otherwise indicated, toany component which performs the specified function of the describedcomponent (i.e., that is functionally equivalent), even though notstructurally equivalent to the disclosed structure which performs thefunction in the herein illustrated exemplary embodiments of theinvention. In addition, while a particular feature of the invention mayhave been disclosed with respect to only one of several embodiments,such feature may be combined with one or more features of the otherembodiments as may be desired and advantageous for any given orparticular application.

What is claimed is:
 1. A test circuit comprising: a first endpoint; asecond endpoint; a first metal layer, comprising a plurality ofsegments; a second metal layer, comprising a plurality of segments; avia layer, comprising a plurality of vias wherein each via is configuredand disposed to connect a segment of the first metal layer to a segmentof the second metal layer; wherein the first metal layer comprises asegment connected to said first endpoint and also comprises a segmentconnected to said second endpoint; and wherein one of the layersselected from the group consisting of the first metal layer, the secondmetal layer, and the via layer is a functional layer, and the remaininglayers of said group are test layers.
 2. The test circuit of claim 1,wherein the via layer is a functional layer, and the first metal layerand second metal layer are test layers.
 3. The test circuit of claim 1,wherein the second metal layer is a functional layer, and the firstmetal layer and the via layer are test layers.
 4. The test circuit ofclaim 1, wherein the first and second endpoints are each comprised of ametal pad.
 5. The test circuit of claim 4, wherein the metal pad issquare, and has a width and length ranging from about 60 microns toabout 100 microns.
 6. The test circuit of claim 3, wherein the segmentsof the first metal layer are 10 percent to 20 percent wider than thesegments of the second metal layer. 7-20. (canceled)